An optical glass or an optical transparent resin is used as an optical device material for optical systems of various cameras such as a camera, a film-integrated type camera and a video camera. Optical glass is excellent in heat resistance, transparency, dimensional stability, and chemical resistance and so on, and there are large variety of materials having various refractive indexes (nD) and Abbe's numbers (νD). However, it has defects that its material cost is high, and in addition, it has poor moldability and low productivity. Especially, since an extremely advanced technique and a high cost are required in order to mold an aspherical lens used for aberration correction, these defects are a great obstacle for a practical use.
Meanwhile, an optical lens composed of an optical transparent resin, especially composed of a thermoplastic transparent resin is used as a lens for cameras currently because it can be produced on a large scale by injection molding, and in addition, it has the advantage that the production of an aspheric lens is easy. Examples of the thermoplastic transparent resins include polycarbonate composed of bisphenol A, polystyrene, poly-4-methylpentene, polymethylmethacrylate and amorphous polyolefins.
However, when using the optical transparent resin as an optical lens, transparency, heat resistance and low birefringence are required in addition to a refractive index and an Abbe's number. Therefore, it has a defect that parts to be used are limited depending on the physical properties balance of the resin. For example, it is unfavorable that, since poly-4-methylpentene has a low heat resistance and polymethylmethacrylate has low glass transition temperature, low heat resistance and a small refractive index, their field to be used are limited, and, since polycarbonate composed of bisphenol A has weak points that its birefringence is large and so on, its parts to be used are limited.
On the other hand, in general, when the refractive index of an optical material is high, lens elements having the same refractive index can be realized with a surface having a smaller curvature, whereby the aberration amount generated on this surface can be reduced, and downsizing and weight reduction of a lens system can be made possible by reducing the number of lenses, by reducing the eccentricity sensitivity of lenses and by reducing the lens thickness. Therefore, a raise in a refractive index is useful.
Moreover, in an optical design of an optical unit, it is known that the chromatic aberration is corrected by using two ore more lenses having different Abbe's numbers mutually in combination with each other. For instance, a lens composed of an alicyclic polyolefin resin having the Abbe's number of 45-60 is used in combination with a lens composed of a polycarbonate resin from bisphenol A (nD=1.59, μD=29) having the low Abbe's number (nD=1.59, νD=29) to correct the chromatic aberration. Therefore, lowering the Abbe's number of an optical material is absolutely necessary.
Among the optical transparent resins which are in practical use, examples of ones having a high refractive index include polycarbonate composed of bisphenol A (nD=1.586, νD=29) and polystyrene (nD=1.578, νD=34). In particular, since the polycarbonate resin of bisphenol A has a high refractive index and is excellent in heat resistance and physical properties, its optical use has been discussed extensively. However, since both the polycarbonate resin of bisphenol A and polystyrene have the weak point where they have a high birefringence index, they have a limit in their use. Therefore, a resin for an optical use which has a high refractive index and a low birefringence index and is excellent in physical properties balance is being widely developed. Especially, in the field of a digital still camera of recent years, accompanied by the raise of the resolution by improvement in the number of pixels, a lens for cameras having a high image formation property and a lower birefringence index is being required.
Examples of means for reducing the birefringence index of the above-mentioned materials include a technique wherein compositions having birefringence indexes whose positive or negative signs are opposite from each other are combined to cancel mutual birefringence indexes with each other. Whether the sign of the birefringence index is positive or negative is determined by the difference between polarizability of the polymer main chain direction and polarizability of the polymer side chain direction. For example, a polycarbonate resin from bisphenol A wherein polarizability of the polymer main chain direction is larger than polarizability of the polymer side chain direction has a positive birefringence index and a polycarbonate resin from bisphenol having a fluorene structure whose polarizability of the polymer side chain direction is larger has a negative birefringence index. Therefore, the component ratio of the composition of these materials having a birefringence index of opposite signs is critically important.
As a method for reducing a birefringence index, there is a report of a bisphenol having a fluorene structure whose polarizability of the polymer side chain direction is larger (Patent Document 1). However, as a result of study by the inventors of the present invention, it was found that the component ratio of the resin composition in said report was insufficient for canceling a positive and negative intrinsic birefringence indexes with each other and hence a material which does not have a low birefringence index as intended was obtained.
In addition, a polycarbonate resin having a fluorene structure is disclosed (Patent Documents 2 and 3). In said document, however, only a photoelastic coefficient of a filmed polycarbonate resin was examined. The birefringence index as a so-called lens molding including both an orientation birefringence and a photoelastic birefringence was not examined.
As a result of study by the inventors of the present invention, actually, it was found that the component ratio of the resin compositions described in said Documents 2 and 3 was insufficient for canceling a positive intrinsic birefringence with a negative intrinsic birefringence and the birefringence index as a so-called lens molding was extremely high. Moreover, these documents does not even study a refractive index and an Abbe's number which are important optical properties for a lens. Furthermore, effects of the inventions disclosed in these documents are based upon the assumption that they are used as optical substrate materials such as an optical disk.
In addition, it is disclosed that a resin composition which comprises a polycondensation or polyaddition polymer having a fluorene compound as a monomer unit and having at least one sulfur atom in a repeating unit, and an optical device which is produced by injection molding of said resin composition (Patent Document 4). As a result of study by the inventors of the present invention, it was found that the resin composition disclosed in said Patent Document, though having a high refractive index of around nD=1.7, was insufficient for canceling a positive intrinsic birefringence with a negative intrinsic birefringence, and hence the birefringence index as a so-called lens molding was extremely high, and the lens molding would be colored caused by low heat resistance and its reduction of a total light transmittance would be significant. Furthermore, it became clear that, when injection molding is carried out repeatedly, the injection molding machine or the mold would be corroded by decomposed gas containing sulfur and an industrial implementation would be difficult.
As mentioned above, an optical lens having a high refractive index, a low Abbe's number, a low birefringence index, a high transparency and a high glass transition temperature (heat resistance) in a well-balanced manner has not been known so far.    Patent Document 1: Jpn. Pat. Publication No. H7-109342    Patent Document 2: Jpn. Pat. Publication No. H10-101786    Patent Document 3: Jpn. Pat. Publication No. H10-101787    Patent Document 4: Jpn. Pat. Publication No. 2001-106761